1. Field
This document relates to a backlight unit and, more particularly, to a backlight unit that minimizes damage to an optical sheet due to external impacts, and a liquid crystal display employing the same.
2. Related Art
In general, currently, an application coverage of a liquid crystal display (LCD) extends thanks to its characteristics that it is lighter, thinner, and driven at a low power consumption. Thus, the LCD is being commonly applied to office automation equipment, audio/video equipment, or the like. The LCD displays a desired image on its screen by controlling the amount of transmission of light beams according to a video signal applied to a plurality of control switches arranged in a matrix form.
Because the LCD is not a self-emissive display device, it requires a light source such as a backlight. The backlight unit for the LCD is divided into a direct type backlight unit and an edge type backlight unit. The edge type backlight unit is configured such that a light source is disposed at an outer side of a flat panel and light from the light source is made incident to the entire surface of a liquid crystal panel by using a transparent light guide plate. The direct type backlight unit is configured such that a light source is disposed on a rear surface of the liquid crystal panel to directly illuminate the entire surface of the liquid crystal panel. Compared with the edge type backlight unit, the direct type backlight unit is advantageous in that it can use a plurality of light sources to enhance the luminance and widen a light emission surface. An LCD TV, for which a large-scale liquid crystal panel is required, commonly employs the direct type backlight unit.
With reference to FIGS. 1 and 2, the direct type backlight unit comprises a light source unit 10 to emit light to the liquid crystal panel, a bottom cover 20 to receive the light source unit 10, support sides 30 installed at both slope faces of the bottom cover 20 and supporting the light source unit 10, and a diffuser 40 and optical sheets sequentially stacked on the light source unit 10.
The bottom cover 20 includes a bottom surface 20a, slope faces 20b extending from the bottom surface 20a in one direction and facing each other, and upper faces 20c extending from the slope faces 20b in one direction and allowing the diffuser 40 and the optical sheets 50 to be stacked thereon. First protrusions 22 are formed on the upper faces 20c of the bottom cover 20 to prevent wobbling of the diffuser 40 and the optical sheets 50 and guide them. Second protrusions 34 are formed on the upper faces 32 of the support sides 30 to also prevent wobbling of the diffuser 40 and the optical sheets 50 and guide them.
The diffuser 40 diffuses light, which is made incident from the light source unit 10, toward a front side of the liquid crystal panel to thereby make a distribution density of light emitted to the liquid crystal panel uniform. The diffuser 40 is guided by the first and second protrusions 22 and 34 so as to be disposed on the upper faces 20c of the bottom cover 20 and the upper faces 32 of the support sides 30.
The optical sheets 50 are to make light, which has been diffused by the diffuser 40, proceed to be perpendicular to the liquid crystal panel to thus enhance light efficiency. The optical sheets 50 comprise one or more diffusion sheets and one or more prism sheets. Further, the optical sheets 50 comprise first ear portions 52 each with a long hole 52a and second ear portions 54 each with a long hole 54a so as to be fixed on the upper faces 20c of the bottom cover 20 and the upper faces 32 of the support sides 30. The first long holes 52a of the first ear portions 52 are fastened with the first protrusions 22 formed on the upper faces 20c of the bottom cover 20, while the second long holes 54a of the second ear portions 54 are fastened with the second protrusions 34 formed on the upper faces 32 of the support sides 30. Here, the first and second long holes 52a and 54a contact with the first and second protrusions 22 and 34 in a point contact manner or with a quite narrow contact area.
As for the LCD having such a backlight unit, if an impact ST is applied from the exterior while the LCD is being transported, the impact ST causes a stress concentration at a point Pi where a linear portion A1 and a curved portion A2 of the first long hole or the second long hole 52a or 54a meet as shown in FIG. 3a. The stress concentration results in a crack at the particular point P1 of the optical sheets 50 as shown in FIG. 3b. The crack generated at the long holes 52a and 54a of the optical sheets 50 hinders the optical sheets 50 from being properly fixed on the upper faces 20c of the bottom cover 20 and on the upper faces 32 of the support sides 30, making them wobble, which results in that debris (particles) generated when the crack was made are introduced to the liquid crystal panel to degrade display quality.